Since the 'Client ID' is a primary key, you cannot change its value from the 'Client Management' window. If necessary, however, you can change a client's ID by following these simple instructions:

a) Select 'Client (Filer) Management' from the 'Module' menu.
b) Right-click on the 'Client ID' you would like to change.
c) Select 'Copy' from the popup menu.
d) If asked if you are creating a 'Correction Client', reply No.
e) Enter the new ID in the 'Client ID' field on the 'Copy Filer' window.
f) Click OK to save your changes.
g) Double-click the new client.
h) Select a tax form to be copied.
i) From the 'File' menu, select 'Import -> Another Client's Returns'.
j) Select the original client and import the returns.
k) Repeat steps (g) through (i) until all returns have been copied.

When done, the new client will contain all of the original client's returns. Once confirmed, you can safely delete the original client.